1. Field of the Invention
The present invention relates generally to systems and methods for closed loop control, and, more specifically, the present invention is directed to a closed loop current controller for an electromagnetic rail gun.
2. Description of the Background
Electromagnetic rail gun technology has been in development for over 50 years. To date, some rudimentary prior art systems have been demonstrated, but the technology has not advanced to where practical systems can as of yet be built. Prior systems have not demonstrated the ability to precisely control the muzzle velocity of a projectile, thereby reducing the control of the ejected projectile. Without precise control of the muzzle velocity, the electromagnetic rail gun can not effectively be used as an artillery device.
The basic principle of a rail gun has been well known for quite some time. Generally speaking, the rail gun is comprised of a pair of parallel rails securely fastened to a structure to prevent them from moving while under force. A low impedance conductive projectile is constructed so that it will slide between the rails while making electrical contact with the rails. For projectile motion and ejection, a large pulse of current is delivered to the rails, and the pulse generates orthogonal electric and magnetic fields behind the projectile. These fields produce a force on the projectile that is directed down the center of the rails. Therefore, the force causes the projectile to accelerate in the direction of the applied force (down the length of the rails) until it is ejected from the rail gun.
The most important requirement of the rail gun is the control of the creation and application of the electromagnetic pulse to the rails. Existing control systems used for electromagnetic rail gun applications use only open loop, feed forward controllers with empirically developed algorithms. These prior systems are based on a defined current profile. However, these open loop controllers do not compensate for variations in the system such as changes in alternator resistances, gun resistance, and projectile mass. The muzzle velocity that is critical to the gun accuracy cannot be controlled with precision for these varying conditions.
Closed loop controllers have not been used on electromagnetic gun applications due to several issues. A discharge event occurs during a 5 to 10 millisecond (ms) period. The required closed bandwidth of the controller necessary to control the gun is between 10,000 rad/s and 15,000 rad/s. Thyristor power converters, used to control the current in electromagnetic rail guns are discrete controllers that have an open loop bandwidth typically at or below this frequency. The control of a thyristor bridge is not linear and can occur only at discrete times, and the number of discrete control events during a discharge is limited to between 6 to 8 events. Because of these constraints, open loop, feed forward predetermined algorithmic control techniques have been used for rail gun applications. This type of control is prone to error when the system operating conditions change.
Alternatives to this open loop, feed forward control approach to electromagnetic rail guns are sought in the artillery arts.